In audio amplifier circuits, variable gain elements are commonly used to control the level or frequency response of an input signal. The variable gain element typically includes a transistor connected to variably attenuate the input signal, and a control circuit for suitably biasing the transistor. FIGS. 1A-1B illustrate various prior art circuits. FIG. 1A shows an open-loop gain control circuit which might be used to implement a tone or volume control, and FIG. 1B shows a closed-loop gain control circuit, which might be used to implement a gain limiter.
Referring to FIGS. 1A-1B, an audio input signal Vin present at terminal 10 is AC-coupled via capacitor 12 to an amplifier comprising the input resistors 14, 16, the operational amplifier 18 and the feedback resistor 20. The inverting input of the amplifier 18 is coupled to the junction of resistors 16 and 20, while the non-inverting input is coupled to a bias voltage Vbias, such as one-half of the supply voltage Vcc. The amplified output signal, Vout, appears at terminal 22. The variable resistance element, bi-polar transistor 24, couples the input signal at junction 15 to the bias voltage Vbias. The conduction of transistor 24, and hence its effective resistance, varies in accordance with a control voltage developed at terminal 26. In the circuit of FIG. 1A, the control voltage is developed open-loop by a variable DC voltage source 28, whereas in the circuit of FIG. 1B, the control voltage is developed closed-loop by a feedback circuit 28 connected to the output of amplifier 18.
While the above-described circuits require only a single control element (transistor 24) and are therefore inexpensive to implement, they exhibit a resistance characteristic that varies non-linearly with input signal strength, tending to limit their usefulness to relatively small input signal voltages. In a practical mechanization, for example, the transistor 24 has been limited to input voltages of 15-20 mV peak-to-peak in order to realize acceptably low output signal distortion. Additionally, the above-described circuits tend to exhibit a relatively abrupt and non-linear response to the control voltage. While this characteristic may be acceptable in certain closed-loop applications, it is undesirable in many open-loop applications.